The present invention relates to apparatuses and methods for performing high-speed electric discharge machining operations. More particularly, the present invention relates to an apparatus and a method for performing an electric discharge machining operation, wherein the apparatus is adaptable determine the hole, or multiple holes, effective area as a function of back pressure gradient and breakthrough for a workpiece.
A gas turbine engine typically includes a core engine having, in serial flow arrangement, a high pressure compressor that compresses airflow entering the engine, a combustor that burns a mixture of fuel and air, and a turbine that includes a plurality of airfoils in both rotating blades and stationary vane assemblies that interact to extract rotational energy from airflow exiting the combustor. The turbine is subjected to high temperature airflow exiting the combustor. Therefore, turbine components are cooled to reduce thermal stresses that may be induced by the high temperature airflow.
The rotating blades and stationary vanes include hollow airfoils that are supplied cooling air through cooling channels that vent through cooling apertures in the airfoil and/or through openings at the top of the airfoil. The airfoils include a cooling cavity bounded by sidewalls that define the cooling cavity. The cooling cavity is partitioned into cooling chambers that define flow paths for directing the cooling air. During airfoil manufacture, the cooling holes in the airfoil are drilled or machined from the outer side of the airfoil to the internal cooling cavity. More specifically, an electric discharge machining process is used to create the cooling apertures into the desired dimensions.
Electric discharge machining (EDM), sometimes referred to as “electro-discharge machining” or “electrode discharge machining”, is a known process for drilling deep, small diameter holes in a metal workpiece, such as a turbine blade or vane of a gas turbine engine. EDM operates on the principle that, if a electrically-charged EDM tool (typically, a negatively-charged copper-alloy electrode) is brought within close proximity to a electrically-charged (for example, positively-charge if the electrode is negatively-charged) workpiece which is sometimes submerged in a bath of dielectric fluid (typically, water), an electric potential difference exists between the electrode and the workpiece and a spark discharge will arc the gap therebetween, thereby eroding a small amount of material from the workpiece adjacent the electrode. If the negative charge to the electrode is in the form of a series of pulse charges, the electric potential difference between the electrode and the workpiece is systematically repeated such that spark discharges occur at a rapid rate, and a hole can be “drilled” into the workpiece if the electrode is incrementally advanced into the workpiece as workpiece material is slowly eroded therefrom. The dielectric fluid assists in the formation of the spark discharges, cools the workpiece during repeated spark discharges and carries away material eroded from the workpiece.
The existing EDM drilling process for creating cooling holes has no instant feedback in terms of airflow flowing through the holes being drilled. The current state of the art does not have means to adjust for core and wall thickness variation of castings to control the in-process airflow within a workpiece. The in-process airflow testing is done after the entire drilling operation is complete. As a result, the in-process airflow test results may deviate from the target values. That, in turn, affects the final airflow capability and causes non-conforming hardware and workpieces. In addition, the current manufacturing process lacks a reliable real time method of machining performance and feedback for the detection of hole breakthrough. Current manufacturing processes and machine designs lack the ability to cut off machining to prevent wall strikes on the interior of the workpiece. The lack of control has become an important consideration given the growing complexity of internal core geometry for workpieces. Prior methods developed to address this aspect of the machining process involved the measurement and feedback of the machining head velocity and/or monitoring and feedback of the voltage from the electrode/head. These prior methods are not reliable due to the complexity of airfoil machining and internal cavities that do not always provide clean breakthrough, and therefore signature analysis becomes difficult.